The present invention relates to an orthodontic device adapted in the field of orthodontics for moving teeth in a prescribed direction for the purpose of correcting irregularities of the teeth, and more particularly to a novel orthodontic bracket.
Generally, orthodontic treatment for moving specified teeth in a prescribed direction is carried out by attaching a bracket firmly to the surface of a tooth being subjected to the orthodontic treatment; retaining an arch wire on the bracket through a ligature; and exerting a corrective force onto the tooth resulting from the bending and tension of the arch wire.
Brackets which have heretofore been used for this purpose generally are made of metal such as stainless steel because such materials satisfy the requirements indispensable to such brackets primarily a mechanical strength high enough to retain an arch wire thereon and physiological inactivity. The appearance of such brackets is a distinct disadvantage, however, as the patient's teeth exhibit a metallic appearance. This drawback has resulted in a search for a bracket that combines the strength of metallic brackets with a more pleasing appearance.
Japanese Patent Public Disclosure No. 60-234656 discloses a bracket formed of a single transparent crystal of monocrystalline alumina. Although this bracket has been proposed as a solution to the esthetic problems encountered by metallic brackets, it also presents several inherent drawbacks. First, this bracket is expensive to produce. The bracket is formed by pulling molten alumina within a crucible made of an expensive material such as iridium, platinum, etc., onto the upper surface of a die made of an similarly expensive material such as iridium, molybdenum, etc.; allowing the molten alumina on the die to grow into an alumina rod; cutting the alumina rod into pieces of a prescribed size; and subjecting each of the alumina pieces to secondary mechanical processing such as machining the bottom surface of the bracket so that it can be bonded to the curved tooth surface.
Not only does the crystal pulling process (known as the Czochralski Process) require the use of expensive equipment, it also entails limited productivity. The velocity of crystal growth into an alumina rod is very slow, inherently precluding a mass production process. Furthermore, the requirement for secondary mechanical processing introduces a number of cumbersome manufacturing steps.
Additionally, this device solves the esthetic problems of metallic brackets, but it introduces a new esthetic drawback of its own. Monocrystalline alumina is a simple crystal having no grain boundary, and it exhibits light transmittance of substantially 100%. When a bracket made of such monocrystalline alumina is mounted on a tooth, the bracket acts as a lens, and the color of the patient's own tooth becomes conspicuous through the transparent bracket when the patient opens his mouth, an appearance often found unpleasant. Thus, a completely transparent bracket does not always enhance the bracket's esthetic qualities, and therefore brackets formed of monocrystalline alumina have not yet gained widespread acceptance.
In addition, the monocrystalline alumina bracket is difficult to install. A metallic bracket is fixed to the surface of a tooth by welding a base member of a comparatively wide area such as wire gauze to the bottom surface of the metallic bracket and then applying a bonding material to the base member. Due to the material involved, one cannot weld such base members to monocrystalline alumina brackets, and therefore such brackets are bonded directly onto the tooth surface. On fixing the monocrystalline alumina bracket to the surface of a tooth, however, the bonding material applied to the bottom surface of the bracket flows outward to the rear side of wings formed on the opposite sides of the bracket and solidifies there. Because the rear sides of the bracket wings define spaces for stopping a ligature, the outflow of bonding material obstructs the installation of the ligature, requiring the orthodontist to remove such material. The necessity of this cumbersome step further militates against the practical use of transparent brackets of monocrystalline alumina.
Thus, the only alternative to metallic brackets offered by the prior art is expensive to manufacture, unpleasant in appearance, and difficult to install. Clearly, a need exists for a non-metallic bracket, pleasing in appearance, that can be produced rapidly and cheaply.